Decline of Pseudodiaptomus hessei (Copepoda, Calanoida) in two water bodies located in the Senegal River hydrosystem (West Africa): Hypotheses and perspectives

Pseudodiaptomus hessei is a key species in many water bodies in the Senegal River hydrosystem but it became rare or completely disappeared from two ecosystems (Lake Guiers and Dakar Bango Reservoir; Senegal, West Africa) after major hydrological changes caused by human action, mainly impoundments on the river in 1985, and the opening of a new estuary mouth in 2003. Kâ et al. [Kâ, S., Pagano, M., Ba, N., Bouvy, M., Leboulanger, C., Arfi, R., Thiaw, O.T., Ndour, E.H.M., Corbin, D., Defaye, D., Cuoc, C., Kouassi, E., 2006. Zooplankton distribution related to environmental factors and phytoplankton in a shallow tropical lake (Lake Guiers, Senegal, West Africa). International Review of Hydrobiology 91(5), 389–405] put forward several hypotheses to explain the reasons for this decline: salinity and chemical changes in the water, predation by a cyclopid predator Mesocyclops ogunnus and/or the inhibiting effects of cyanobacteria and/or diatoms (allelopathy). This study assessed these hypotheses by studying the distribution of P. hessei in 13 stations (including the Dakar Bango reservoir and Lake Guiers) in relation to physical, chemical and biological (phyto- and zooplanktons) factors at each station. We produced a distribution pattern for this species in the Senegal River hydrosystem. Rank correlations and principal component analysis showed that P. hessei was not correlated with conductivity but was positively correlated with pH and alkalinity, suggesting chemical effects. The clear association with two filamentous cyanobacteria (Cylindrospermopsis raciborskii and Anabaena sp.) did not appear to support the hypothesis of cyanobacteria inhibition but blooming conditions were never encountered during the study. Negative correlation with diatoms (especially with Fragilaria sp.) suggested that aldehyde producing diatoms had a negative effect. Negative correlations with cyclopids such as Mesocyclops supported the hypothesis of cyclopid predation to explain the decline of P. hessei. This study proposes several lines of research for future studies to test these hypotheses.     

Contribution of the Bacterial and Microphytobenthic Microflora in the Energetic Demand of the Meiobenthos in an Intertidal Muddy Sediment (Kerguelen Archipelago)

Abstract. An intertidal mud sediment in the Kerguelen Archipelago was sampled weekly from January 1982 to March 1983. Measurements were taken for organic matter (carbon and nitrogen) and biological (bacteria, microphytobenthos, and mciofauna) parameters. The sediment is rich in organic matter, with a mean organic carbon content of 130gC- m-². Benthic living components yield low mean biomasscs in relation to the total organic pool. Besides some marine inputs, the main allochthonous source was identified as terrestrial inputs. Benthic activities seemed strongly governed by food availability and not by temperature. Obvious seasonal changes in bacterial abundances (direct counts), primary production, and mciofauna distributions were observed. Photosynthctic production reached high values (up to 2275 mg C-m^-2-d^-1), reflecting the presence of a dense microphytobenthos. Preliminary energy budgets for two seasons (spring and winter) have been constructed. In both periods, the measured primary production can support the high mciofauna production.     

Do human activities affect the picoplankton structure of the Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia)?

The spatial variations of the picoplankton (photoautotrophic and heterotrophic microorganisms) in the Ahe atoll lagoon were studied in May and October 2008 to assess whether they were affected by human activities along the atoll. Spatial patterns were studied using 10 sampling stations chosen according to the location of the anthropogenic activities (pearl farming, harbor). Experiments were also carried out to determine whether bacterial growth, with or without predators, was limited by inorganic (N and P) substrates. The results showed that heterotrophic bacterioplankton abundance was superior to the photoautotrophic organisms, especially in May. Significant increases in bacterial abundance were observed in May after 24h incubation with +P and +N (but not in October). All samples complied with the quality levels for fecal indicator bacteria (FIB) defined by the European Union and there was no evidence that human sewage had any impact on picoplankton over the whole atoll.     

Estuarine microbial community characteristics as indicators of human-induced changes (Senegal River,West Africa)

Understanding how human-induced management interacts with and affects the structure and functioning of large estuarine ecosystems is a major research challenge. In West Africa, human intervention on the Senegal River Estuary was intended to reduce the impact of major flooding by opening a new mouth in October 2003, 25 km to the north of the existing mouth. This study describes the effects of the new environmental conditions on the physical and biochemical characteristics of the water column and on microbial communities (bacteria, phytoplankton by size class and heterotrophic nanoflagellates (HNF)) in comparison with the situation in 2002. In 2006, seventeen stations were sampled, during both neap and spring tides, at a depth of 0.5 m along a salinity gradient from freshwater to marine conditions. Inorganic nutrient levels were often low but there were high levels of chlorophyll a in the estuarine area (mean of 13.7–20.7 μg L⁻¹ in spring and neap tide conditions, respectively) producing a eutrophic status in this estuary. Average HNF abundances were lower (mean of 108 and 174 cells l⁻¹ during neap and spring tides, respectively) compared to the situation in 2002 (mean between 2.5 and 6.7 × 10⁴ cells l⁻¹). Three biological indicators for assessing environmental changes are discussed: ratio of bacteria to heterotrophic flagellate abundances, ratio of picophytoplankton to nanophytoplankton, and the density of thermo-tolerant coliforms (TTC) and faecal streptococci. It is demonstrated that man-made alteration of the hydrologic regime can modify the microbial community structure and cause the health status of the estuary to deteriorate.     

Virioplankton distribution and activity in a tropical eutrophicated bay

The study of lysogeny in aquatic systems is an often overlooked aspect of microbial ecology, especially in tropical environments. Herein, the fraction of lysogenized cells (FLC) was detected in the surface waters of 20 coastal stations distributed from the eutrophicated shoreline to seaward waters of Hann Bay (Senegal). Concurrently, viral lytic infection rates were extrapolated from the frequency of visibly infected bacterial cells (FVIC), as determined from transmission electron microscopy observations. The experimental induction of prophage was observed in less than 3% of indigenous marine bacteria, suggesting that lysogenic stages of infection are rare in Hann Bay. Similarly, only 0.5–4.7% of bacteria showed visible signs of lytic infection. However, the positive correlation between the fraction of lysogenic and lytic cells (r = 0.67, p < 0.05, n = 20) may actually indicate that the coexistence of both lifestyles may be due to the massive and rapid induction of lysogens, potentially from the high levels of local UV radiation. Overall, we suggest that the determination of FVIC and FLC to examine the predominance of one type of cycle versus the other may be a source of misinterpretation in some particular aquatic environments.     

Water quality and health status of the Senegal River estuary

The Senegal River estuary was sampled in May 2002 to get the first data on both the trophic and sanitary status of the water of the main river of the northwest African coast. Several physical, chemical and microbiological variables were measured twice along a transect. Inorganic nutrient concentrations were low while phytoplanktonic abundances (0.58-1.8 x 10(5) cells ml(-1)), bacterial abundances (0.27-8.1 x 10(7) cells ml(-1)), activity (22-474 pmol l(-1) h(-1)), were among the highest recorded in such ecosystems. Microbiological variables revealed a eutrophicated status for this estuary. Largest abundances of fecal contamination bacterial indicators were only detected in localized areas (Saint-Louis city and surrounding areas). The apparent good survival of fecal indicator bacteria in the estuarine waters despite a long residence time (4-5 days) has been evaluated by complementary survival experiments. Exposed to a salinity gradient, a local Escherichia coli strain showed a significantly better survival than those of an E. coli reference strain.     

Uncoupled viral and bacterial distributions in coral reef waters of Tuamotu Archipelago (French Polynesia)

This study examined the distribution of virioplankton and bacterioplankton in two coral reef systems (Ahe and Takaroa atolls) in the Tuamotu Archipelago, in comparison with the surrounding oligotrophic ocean. Mean concentrations of 4.8×10(5) and 6.2×10(5)cellsml(-1) for bacteria and 8.1×10(6) and 4.3×10(6)VLP(virus-like particle) ml(-1) were recorded in Ahe and Takaroa lagoons, respectively. Chlorophyll-a concentrations and dissolved organic matter were higher in Ahe whereas (3)H thymidine incorporation rates were higher in Takaroa. First data on lytic and lysogenic strategies of phages in coral reef environments were discussed in this paper. The fraction of visibly infected cells by viruses was negligible regardless of the lagoon station (mean=0.15%). However, the fraction of lysogenic cells ranged between 2.5% and 88.9%. Our results suggest that the distribution patterns of virioplankton are apparently not coupled to the spatial dynamics of the bacterioplankton communities.     

Pearl oysters Pinctada margaritifera grazing on natural plankton in Ahe atoll lagoon (Tuamotu archipelago, French Polynesia)

In atoll lagoons of French Polynesia, growth and reproduction of pearl oysters are mainly driven by plankton concentration. However, the actual diet of black-lip pearl oysters Pinctada margaritifera in these lagoons is poorly known. To fill this gap, we used the flow through chamber method to measure clearance rates of P. margaritifera in Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia). We found: (i) that pearl oysters cleared plankton at a rate that was positively related to plankton biovolume, (ii) that nanoflagellates were the main source of carbon for the pearl oysters, and (iii) that the quantity and origin of carbon filtrated by pearl oysters was highly dependent on the concentration and composition of plankton. These results provide essential elements for the comprehension of growth and reproduction variability of pearl oysters in atoll lagoons of French Polynesia.     

Use of inhibitors for coastal bacteria and phytoplankton: Application to nitrogen uptake measurement

For several decades, prokaryotic and eukaryotic inhibitors have been used to exclude bacteria from microalgal cultures and for investigating prey-predator relationships. Recently there has been considerable interest in using specific inhibitors for studying the interactions between bacteria and phytoplankton, by selective repression of either organism’s activity. The effectiveness of chemical inhibitors must be tested before applying them to natural communities to partition metabolic activities between functional groups. Six different antibiotics selected from the most commonly reported in the literature were tested, at concentrations varying from 12.5 to 100 mg L⁻¹, for their effect on bacterial growth and functional diversity of natural communities from Mediterranean coastal waters. Penicillin and streptomycin each at a final concentration of 100 mg L⁻¹ significantly reduced bacterial growth within 2 h. There was a greater impact on bacterial functional diversity when both antibiotics were mixed together. This mixture did not have any significant effect on the growth of selected cultured phytoplankton strains, whereas the eukaryote inhibitor cycloheximide at 100 mg L⁻¹ reduced growth within 2 h of incubation. The penicillin–streptomycin mixture and cycloheximide alone successfully partitioned NH₄⁺ and NO₃⁻ uptake between bacteria and phytoplankton bi-weekly sampled in a coastal lagoon in Autumn, where bacterial contribution to total NH₄⁺ and NO₃⁻ uptake averaged 46 and 41%, respectively. The use of specific inhibitors may be a valuable method for studying interactions, such as competition and mutualism, or lack of interaction between the different components of microbial communities and could be used to study their relative importance in biogeochemical fluxes.     

Metazooplankton communities in the Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia): Spatiotemporal variations and trophic relationships

Metazooplankton abundance, biomass (<80μm, 200-500μm and >500μm) and community structure in the Ahe atoll were studied together with their relationships with environmental factors (temperature, salinity, wind) and trophic factors (phytoplankton, bacteria, heterotrophic nanoflagellates (HNF) and ciliates) during three periods in 2008-2009. Meroplankton, mainly bivalve and gastropod larvae, was dominant. Holoplankton was dominated by copepods, the main species being Oithona spp., Paracalanus parvus, Clausocalanus spp., Corycaeus spp., Acartia fossae and Undinula vulgaris. The results suggest a clear wind influence on the structure and horizontal distribution of the zooplankton communities. The metazooplankton appeared to be controlled mainly by food resources, suggesting a bottom-up control. The low nanophytoplankton biomass in contrast to the high abundance of picophytoplankton, HNF and nano-particle grazers (mainly Oithona spp., Paracalanus and bivalve larvae) highlighted the importance of the microbial loop in the food web.